Caption: Rectal temperature. A child suffering fever has his rectal temperature taken. Fever, also known as pyrexia, is defined as a body temperature above 37 Celsius (98.6 Fahrenheit) if measured in the mouth, or 37.7 (99.9) if measured in the rectum. Most fevers are caused by bacterial or viral infections and may be accompanied by other symptoms such as shivering, sweating, thirst and hot skin.